Method of cleaning gases containing organic pollutants

ABSTRACT

A method of cleaning gases includes introducing a gas containing organic pollutants into a scrubber and providing a scrubbing liquid. Finely divided solids capable of remaining substantially in suspension in the scrubbing liquid are introduced into the scrubbing liquid and the gas is wet scrubbed to remove the organic pollutants as dissolved gases and as particulate. A reagent capable of reacting, in combination with the finely divided solids, with the organic pollutants removed from the gas, is introduced into the scrubbing liquid to convert the removed pollutants into different compounds. A major portion of the scrubbing liquid is reused in the wet scrubbing.

FIELD OF THE INVENTION

The present invention relates to methods of wet cleaning gasescontaining condensable pollutants, in particular organic pollutants.

HISTORY OF THE RELATED ART

For example in the case of household garbage incineration fumes, on themost modern incinerations after recovery of energy, i.e. at atemperature of about 180° to 250° C., gaseous organic pollutantcontents, most often lying in the range 1 to 10 mg/m³, may be measured(measurement of the total organic carbon by the flame ionizationmethod). Among these organic pollutants, certain compounds form thesubject matter of particular investigations due to their inherenttoxicity: this is the case for polychlorodibenzo-p-dioxines (PCDD) andpolychlorodibenzofuranes (PCDF) for which it is necessary to attain verylow contents.

In order to collect these pollutants, several techniques have beenproposed. The first technique consists of injecting in the gases anactive carbon powder, ensuring a sufficient contact time between thepowder and the gases, and then separating this active carbon powder likefly ash, in an electro-filter or in a sleeve filter. This method may beusefully combined with a semi-dry cleaning or a dry cleaning; in thelatter case, the active carbon powder may be mixed with slaked limepowder before injection in the gases. This first technique is, however,limited in its application by several factors; the temperature of thegases must remain fairly high to avoid clogging and corrosion bycondensation, which limits the possibilities of absorption; the presenceof other pollutants consumes the absorption potential of the activecarbon powder; and, finally, the mixture of the active carbon powderwith the fly ash limits the maximum quantity of active carbon powderwhich may be injected.

In order to avoid the drawbacks and limitations of this first technique,a second technique has been proposed. It consists of cleaning the gasesfirstly down to very low values for the principal particulate andgaseous pollutants then in passing the gases to a filter comprisingseveral layers of active carbon in granular form. This second techniqueinvolves the production of large complementary installations and it,too, is limited in its application by several factors; the temperatureof the gases must not be too high due to the risks of inflammation ofthe large quantity of active carbon employed; it is necessary to renewthe active carbon fairly often by reason of the presence of otherpollutants which are absorbed and to limit the risks of inflammation;and, finally, the active carbon must, after use, be treated due to itscomposition.

In order to avoid these drawbacks and limitations, a third technique hasbeen proposed. It consists of cleaning the gases firstly down to verylow values for the principal particulate and gaseous pollutants exceptfor the nitrogen oxides then in passing the gases to a filterconstituted by catalysts identical to those used in the known processesof selective catalytic reduction of nitrogen oxides; An injection of NH₃gas is performed for the selective reduction of the nitrogen oxides, butthe quantity of catalysts is increased to allow the absorption anddestruction of the dioxines and furanes: A variant consists of addingcatalysts specifically designed for the destruction of the dioxines andfuranes. This third technique involves the production of largecomplementary installations and it, too, is limited in its applicationby several factors: the temperature of the gases must remain fairly highto avoid poisoning of the catalysts by condensation of salt remaininggaseous if the temperature is fairly high; the prior cleaning of thegases must be thorough in order to avoid poisoning of the catalysts byspecific elements, arsenic for example; finally, although this thirdtechnique has the advantage of destroying the dioxines and furanes,there is a risk of desorption before destruction under the effect ofsuch or such upstream factor which is more or less well controlled.

Furthermore, the cleaning systems designed for the separation of theother pollutants may have a role--poorly controlled--with respect tothese condensable organic pollutants; in dry and semi-dry systems withan electro-filter or filter with a sleeve, there is generally, bycatalytic reaction, an increase of particular pollutants such asdioxines and furanes; and in the wet systems, there may be an increaseor decrease, without, however, systematically obtaining extremely lowresidual contents.

BRIEF DESCRIPTION OF THE DRAWING

The drawing FIGURE is a flow diagram of a preferred embodiment of themethod in accordance with the invention.

SUMMARY OF THE INVENTION

The present invention aims at overcoming the above-described drawbacksand limitations by providing a method of cleaning to ensure thecollection of the condensable organic pollutants as well as thedestruction thereof.

To that end, the gases are cleaned by a wet method which, according tothe state of the art, firstly ensures soaking of the gases whichprovokes condensation of the condensable organic pollutants on the dustparticles present in the gases or in the form of new fine particles,then the collection of all of these particles by transfer in thescrubbing liquid. According to the invention, the wet cleaning method ischaracterized by:

A) the introduction in the scrubbing liquid of an intermediate phase(for example in the form of a finely divided solid) immobilizing thecondensable organic pollutants collected in order to avoid the pllutantsbeing released again into the gases while awaiting the end of thereaction of destruction according to B) hereinafter and,

B) the employment in the scrubbing liquid itself, at the level of theintermediate phase according to A) hereinabove, of reactions ofdestruction of condensable organic pollutants (by oxidation for example)possibly catalyzed by the intermediate phase according to A)hereinabove.

DETAILED DESCRIPTION OF THE INVENTION AND BRIEF DESCRIPTION OF DRAWING

The invention will now be described in detail in connection with FIG. 1which gives a schematic representation of the method forming the subjectmatter of the invention.

The gases (1) to be cleaned are cleaned by a wet method which, accordingto the state of the art, ensures separation of the gaseous andparticulate pollutants at the level of cleaning sought after for thecleaned gases (3). Such a wet method is employed using to a scrubber (2)where the gases are contacted by a recycled scrubbing liquid (11) sentvia a pump (12) into sprays (13) which distribute the recycled scrubbingliquid in the gases. The recycled scrubbing liquid (11) atomized in thescrubber (2), after contact with the gases is recovered (4) in a tank(5) which is often arranged in the base of the scrubber (2). A smallportion (6) of the recovered scrubbing liquid (4) is directed to atreatment station (7). The collected pollutants are separated in theform of solid residue (9) while the clear water (8) is rejected in thenatural medium or recycled in the tank (5). An addition of water (10)compensates for consumptions, in particular due to the evaporation inthe scrubber (2). In general, the contact between the gases and thescrubbing liquid (10) brings these gases to conditions close towater-vapour saturation. The gases are thus soaked and their temperaturegenerally becomes very rapidly slightly higher than the wet temperatureof the gases (1) to be cleaned. This results in a condensation of thecondensable organic pollutants on the dust particles present in thegases (1) or in the form of new fine particles. The scrubber (2) isdimensioned to ensure collection of these particles to attain the levelof cleaning required for the cleaned gases (3). Experience shows thatthe particles collected by transfer in the scrubbing liquid remain inthe liquid in the case of inorganic pollutants but for numerous organicpollutants, there is a tendancy for release in the cleaned gases (3),such release being greater as the concentration in the scrubbing liquidis high. The obtaining of a high level of cleaning imposes aconsiderable flowrate of scrubbing liquid (10) which it is not possiblefor technical and economical reasons to treat like the recoveredscrubbing liquid (6). According to the invention, an intermediate phaseis constituted in the recycled scrubbing liquid (11) by injectingmaterial (14) for example in the tank (5) which material is capable ofimmobilizing the condensable organic pollutants collected. The material(14) may be a finely divided solid such as for example active carbonpowder. To determine the quantity of material (14) to be injected, thecontent of non-immobilized organic pollutants is assessed (15) bymeasuring for example the total organic carbon content of the liquidphase. This assessment may be made either periodically and the rate ofinjection is adjusted manually a priori, taking a reasonable safetymargin, or continuously and the rate of injection is automaticallyadjusted. However, it is also necessary to destroy the collected andimmobilized organic pollutants if it is not desired to attain excessiveconsumptions of the material (14). To that end, a reaction ofdestruction of these organic pollutants is carried out in the scrubbingliquid itself, at the level of-the intermediate phase by introducing areagent (16) which may react with the organic pollutants. In general,the most appropriate reaction is a reaction of oxidation, for examplewith hydrogen peroxide H₂ O₂, and which may benefit from a catalyticeffect of the intermediate phase. To determine the quantity of reagent(16) to be injected, its real content in the liquid is assessed (17).For example, in the case of a reaction of oxidation, the dissolvedoxygen of the liquid phase is measured. This assessment may be madeeither periodically and the rate of injection is adjusted manually apriori, taking a reasonable safety margin, or continuously and the rateof injection is automatically adjusted. In the case of a reaction ofoxidation, it may be desirable, instead of consuming a reagent such ashydrogen peroxide, to use oxygen dissolved naturally in the scrubbingliquid during its contact with the gases. Injection of reagent (16) isthen obtained by this dissolution and it is the flow of scrubbing liquidwhich is then adjusted to maintain the real dissolved oxygen at therequired level. In the same manner, in the case of gases produced fromprocesses of combustion, it is possible for this process to work so thatthe intermediate phase is constituted by the dusts emitted and containedin the gases (1), in particular the unburnt products. It may also bepreferable, from a practical point of view, to inject the material (14),not in the tank (5), but in the gases (1) to be cleaned, upstream of thescrubber (2).

The quantities and natures of the condensable organic pollutants varyfrom one site of application to another, and it is necessary for eachsite to associate, experimentally, the content of condensable organicpollutants in the cleaned gases (3) with the assessments (15 and 16).

The following is an example of the use of the method of the invention.For household garbage incineration gases, the method of the inventionwas carried out on a scrubber to achieve a very thorough cleaning on thefollowing pollutants:

dusts--less than 1 mg/Nm³

HCl--less than 1 mg/Nm³

SO₂ --less than 20 mg/Nm³

For the condensable organic pollutants, attention was given to thespecific toxic pollutants, dioxines and furanes.

By injecting 20 kg/h of active carbon powder having a specific surfaceof 1000, the following was obtained:

a rate of destruction greater than 90%

a collection output (with oxidation by the gases) greater than 99%

a collection output (with oxidation with hydrogen peroxide) greater than99.8%.

It must, moreover, be understood that the foregoing description has beengiven only by way of example and that it in no way limits the domain ofthe invention which would not be exceeded by replacing the details ofexecution described by any other equivalents. This is why, insofar assuch changes, modifications or re-arrangements are obvious for a manskilled in the art, they are considered as belonging totally to thepresent invention.

We claim:
 1. A method of removing organics contained in combustion gasescomprising the steps of:providing combustion gases containing organics;introducing finely divided solids selected from the group consisting ofpowders of activated carbon, titanium oxide, alumina, iron oxide andsilica into said combustion gases; providing a scrubber and a scrubbingliquid; introducing said combustion gases and said scrubbing liquid intosaid scrubber, said finely divided solids remaining substantially insuspension in said scrubbing liquid during movement of said scrubbingliquid; wet scrubbing said combustion gases with said scrubbing liquidto remove said organics; introducing a reagent selected from the groupconsisting of hydrogen peroxide and ozone directly into said scrubbingliquid, said reagent reacting, in combination with said finely dividedsolids, with said organics removed from said gases, to oxidize theremoved organics; reusing directly in said wet scrubbing a major portionof said scrubbing liquid having said finely divided solids suspendedtherein; and bleeding a small portion of said scrubbing liquid to aneffluent treatment plant.
 2. The method of claim 1, wherein said smallportion of said scrubbing liquid contains said finely divided solids. 3.The method of claim 1, wherein said finely divided solids comprise ironoxide powder.
 4. The method of claim 1, wherein said finely dividedsolids comprise activated carbon powder.
 5. The method of claim 4,wherein said reagent is ozone.
 6. The method of claim 4, furthercomprising the step of measuring the content of dissolved organics insaid scrubbing liquid and adjusting the introduction of said activatedcarbon powder into said scrubbing liquid, the amount of said activatedcarbon powder introduced being increased as said content of dissolvedorganics in said scrubbing liquid is increased.
 7. The method of claim4, further comprising the step of measuring the content of said reagentin said scrubbing liquid and adjusting the introduction of said reagentinto said scrubbing liquid, the amount of reagent introduced beingincreased as said content of said reagent in said scrubbing liquid isdecreased.
 8. The method of claim 4, further comprising the steps ofmeasuring the content of dissolved organics in said scrubbing liquid andadjusting the introduction of said activated carbon powder into saidscrubbing liquid, the amount of said activated carbon powder introducedbeing increased as said content of dissolved organics in said scrubbingliquid is increased, and measuring the content of said reagent in saidscrubbing liquid and adjusting the introduction of said reagent intosaid scrubbing liquid, the amount of reagent introduced being increasedas said content of said reagent in said scrubbing liquid is decreased.9. A method of removing organics contained in combustion gases,comprising the steps of:introducing combustion gases containing organicsinto a scrubber; providing a scrubbing liquid; introducing separatelyfrom said combustion gases finely divided solids selected from the groupconsisting of powders of activated carbon, titanium oxide, alumina, ironoxide and silica directly into said scrubbing liquid, said finelydivided solids remaining substantially in suspension during movement ofsaid scrubbing liquid; wet scrubbing said combustion gases with saidscrubbing liquid to remove said organics; introducing separately fromsaid combustion gases a reagent selected from the group consisting ofozone and hydrogen peroxide into said scrubbing liquid in said scrubber,the reagent reacting, in combination with said finely divided solids,with said organics removed from said combustion gases, to oxidize theremoved organics; reusing directly in said wet scrubbing a major portionof said scrubbing liquid having said finely divided solids suspendedtherein; and bleeding a small portion of said scrubbing liquid to aneffluent treatment plant.
 10. The method of claim 4, wherein said finelydivided solids comprise activated carbon powder, and further comprisingthe step of measuring the content of said dissolved organics in saidscrubbing liquid and adjusting the introduction of said activated carbonpowder into said scrubbing liquid, the amount of activated carbon powderintroduced being increased as said content of said dissolved organics insaid scrubbing liquid is increased.
 11. A method of removing organicpollutants contained in gases, comprising the steps of:introducing gasescontaining organic pollutants into a scrubber; providing a scrubbingliquid; introducing finely divided solids into said scrubbing liquid,said finely divided solids remaining substantially in suspension duringmovement of said scrubbing liquid; wet scrubbing said gases with saidscrubbing liquid to remove said organic pollutants; introducing areagent into said scrubbing liquid, said reagent reacting, incombination with said finely divided solids, with said organicpollutants removed from said gas, to convert the removed organicpollutants into different compounds; reusing directly in said wetscrubbing a major portion of said scrubbing liquid having said finelydivided solids suspended therein; and bleeding a small portion of saidscrubbing liquid to an effluent treatment plant.
 12. The method of claim11, wherein the reaction is an oxidation reaction.
 13. The method ofclaim 11, wherein said finely divided solids and said reagent areintroduced into said scrubbing liquid in said scrubber.
 14. A method ofremoving organics contained in combustion gases, comprising the stepsof:introducing combustion gases containing organics into a scrubber;providing a scrubbing liquid; introducing separately from saidcombustion gases finely divided solids selected from the groupconsisting of powders of activated carbon, titanium oxide, alumina, ironoxide and silica into said scrubbing liquid in said scrubber, saidsolids remaining substantially in suspension during movement of saidscrubbing liquid; wet scrubbing said combustion gases with saidscrubbing liquid to remove said organics; dissolving oxygen into saidscrubbing liquid in said scrubber, the dissolved oxygen reacting, incombination with said finely divided solids, with said organics removedfrom said combustion gases, to oxidize the removed organics; reusingdirectly in said wet scrubbing a major portion of said scrubbing liquidhaving said finely divided solids suspended therein; and bleeding asmall portion of said scrubbing liquid to an effluent treatment plant.15. The method of claim 14, wherein said small portion of said scrubbingliquid contains said finely divided solids.
 16. The method of claim 14,wherein said finely divided solids comprise activated carbon powder. 17.The method of claim 16, wherein said combustion gases contain oxygen andoxygen is dissolved in said scrubbing liquid from said combustion gases.18. The method of claim 17, wherein said combustion gases are wasteincineration gases.
 19. The method of claim 14, wherein said combustiongases contain oxygen and oxygen is dissolved in said scrubbing liquidfrom said combustion gases.
 20. The method of claim 14, furthercomprising the step of measuring the content of dissolved organics insaid scrubbing liquid and adjusting the introduction of said finelydivided solids into said scrubbing liquid, the amount of finely dividedsolids introduced being increased as said content of dissolved organicsin said scrubbing liquid is increased.
 21. The method of claim 14,further comprising the step of measuring the content of said dissolvedoxygen in said scrubbing liquid and adjusting the flow of said scrubbingliquid, the flow of said scrubbing liquid being increased as saidcontent of dissolved oxygen in said scrubbing liquid is decreased. 22.The method of claim 14, further comprising the step of measuring thecontent of dissolved organics in said scrubbing liquid and adjusting theintroduction of said finely divided solids into said scrubbing liquid,the amount of said finely divided solids introduced being increased assaid content of dissolved organics in said scrubbing liquid isincreased, and measuring the content of said dissolved oxygen in saidscrubbing liquid and adjusting the flow of said scrubbing liquid, theflow of said scrubbing liquid being increased as said content of saiddissolved oxygen in said scrubbing liquid is decreased.
 23. The methodof claim 14, wherein said solids are introduced in said combustion gasesupstream of said scrubber.
 24. A method of removing organics containedin combustion gases, comprising the steps of:providing combustion gasescontaining organics; introducing finely divided solids selected from thegroup consisting of powders of activated carbon, titanium oxide,alumina, iron oxide and silica into said combustion gases; providing ascrubber and a scrubbing liquid; introducing said combustion gases andsaid scrubbing liquid into said scrubber, said finely divided solidsremaining substantially in suspension in said scrubbing liquid duringmovement of said scrubbing liquid; wet scrubbing said combustion gaseswith said scrubbing liquid to remove said organics; dissolving oxygeninto said scrubbing liquid in said scrubber, said dissolved oxygenreacting, in combination with said finely divided solids, with saidorganics removed from said combustion gases, to oxidize the removedorganics; reusing directly in said wet scrubbing a major portion of saidscrubbing liquid having said finely divided solids suspended therein;and bleeding a small portion of said scrubbing liquid to an effluenttreatment plant.
 25. The method of claim 24, wherein said finely dividedsolids comprise activated carbon powder and said combustion gasescontain oxygen, and further comprising the step of measuring the contentof dissolved oxygen in said scrubbing liquid and adjusting the flow ofsaid scrubbing liquid, the flow of said scrubbing liquid being increasedas said content of said dissolved oxygen in said scrubbing liquid isdecreased.
 26. The method of claim 24, wherein said finely dividedsolids comprise activated carbon powder, and further comprising thesteps of measuring the content of said dissolved organics in saidscrubbing liquid and adjusting the introduction of said activated carbonpowder into said scrubbing liquid, the amount of said activated carbonpowder introduced being increased as said content of said dissolvedorganics in said scrubbing liquid is increased, and measuring thecontent of dissolved oxygen in said scrubbing liquid and adjusting theflow of said scrubbing liquid, the flow of said scrubbing liquid beingincreased as said content of said dissolved oxygen in said scrubbingliquid is decreased.
 27. The method of claim 24, wherein said reagentcomprises hydrogen peroxide or ozone.
 28. The method of claim 24,wherein said combustion gases contain oxygen which is dissolved in saidscrubbing liquid.